Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives an inbound RF signal via the antenna and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with one or more local oscillations to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. The filtering stage filters the baseband signal or the IF signal to attenuate unwanted out of band signals to produce a filtered signal. The data recovery stage recovers raw data from the filtered signal in accordance with the particular wireless communication standard.
The local oscillations used in both the transmitter and receiver may be produced by the same or different local oscillation generators. In either case, a local oscillator generator is typically implemented using a fractional N-synthesizer. As is known, a fractional N-synthesizer has a phase lock loop (PLL) topology that allows for fractional adjustments of the feedback oscillation via a feedback fractional N divider. As is also known, the fractional adjustments of the fractional N divider, allow for fine tuning of the local oscillation such that, for example, a particular channel may be tuned, a particular intermediate frequency may be achieved, et cetera.
While the PLL is readily used in RF transceiver architectures, its accuracy is limited by the linearity of the components comprising the phase locked loop. As is known, a phase locked loop typically includes a phase/frequency detector, a charge pump, a low pass filter, a voltage controlled oscillator, and a divider, which may be a fractional-N divider. In most PLLs, the charge pump is a tri-state device providing a positive current when the output frequency and/or phase is too low, a negative current when the output frequency and/or phase is too high and zero current at all other times. To produce the zero current state, the charge pump activates its two current sources to provide equal, but opposite, currents to the low pass filter. In an ideal environment, the net current provided by the charge pump during the zero current state is exactly zero. In practice, however, the currents produced by each current source of the charge pump are not identical due to integrated circuit manufacturing process variations, temperature variations, etc.
The difference in currents during the off current state of the charge pump results in a non-net zero current being provided to the low pass filter, which results in unwanted spurs in the output oscillation. For example, a translational loop transmitter configured in accordance with the GSM cellular telephony standard includes a baseband modulator, a crystal reference, a phase-and-frequency detector (PFD), a charge pump (CP), a low pass filter (LPF), a voltage controlled oscillator (VCO), a local oscillator (OSC), and two sets of mixers. The phase modulated baseband data is generated in Cartesian form by the host processor and translated to a 25 MHz intermediate frequency by a down-converted version of the transmitted signal. The PFD compares this signal against a fixed 25 MHz reference signal and generates an output proportional to the difference between this fixed reference clock and the output of the summing node. The action of the phase locked loop is to drive this difference to zero; hence, after a brief transient period, the frequency and phase of the transmitted signal equals the frequency and phase of the baseband signal. The LPF is typically chosen such that the closed loop magnitude response has a 3 dB bandwidth of 1-3 MHz. The VCO operates in the desired transmit band (TX band); for GSM this band is 880 MHz-915 MHz. For comparison, the receive band (RX band) for GSM is 925 MHz-960 MHz.
The GSM cellular telephony standard defines limitations to the tolerable level of spurious emission produced by the transmitter when in operation. In particular, emission in the RX band is critical. A problem of significance related to spurious transmission from translational loops is the non-linear operation of the CP, as previously discussed. For instance, a 2.5% mismatch between the current sources of the CP produces spurs during transmission of data that are within the RX band to a degree impermissible by the GSM standard.
As is further known, phase locked loops are used in a variety of applications in radio transceivers, audio equipment, video equipment, etc. In each of the various PLL applications, the above mentioned non-linearities are present and may adversely affect the performance of the electronic equipment incorporating the PLL.
One known effort to combat the current mismatch produced by the charge pump is to use matched components to create the current sources of the charge pump. While this mitigates the problem, it does not reduce it sufficiently enough for today's high performance radio frequency integrated circuit applications, including GSM radios, and other advanced technology applications.
Therefore, a need exists for a highly linear charge pump for use in phase locked loops, where such phase locked loops may be used in radio frequency integrated circuits and other advanced technology applications.